Mobile devices, such as mobile phones, benefit from specific standards that allow devices made by different manufacturers to be compatible with one another through a standardized communication network. The Global System for Mobile Communications (GSM) is one such standard promulgated by the European Telecommunications Standards Institute (ETSI) and employed by the mobile phone industry throughout the world.
FIG. 1 shows a conventional system in which wireless signals 100 are transmitted between a base station 102 and mobile devices 104A, 104B, and 104C, which may be identical or similar devices that are operating under various conditions as described below. The mobile devices 104A, 104B, and 104C represent cellular phones, wireless media devices, or other devices capable of receiving and/or transmitting a radio or other wireless signals 106. The mobile devices 104A, 104B, and 104C may additionally or alternatively be personal digital assistants (PDAs), portable computing devices capable of wireless communication, media player devices, portable gaming devices, and/or wireless access points (WAPs). As shown with reference to mobile device 104A, the mobile devices have various components, which are generally known in the art and include an antenna 106, antenna components 108, processing circuitry 110 and a battery 112. The antenna components 108 include at least one power amplifier to generate a desired power for the signal to be transmitted. The antenna components 108 or the processing circuitry 110 may include an adaptive power control (APC) to control the output of the power amplifier associated with the antenna components. Mobile devices 104A, 104B, and 104C may also have a display 114, keypad, 116, microphone 118, and/or speaker 120.
Any signals 100 sent from the mobile devices (e.g. 104A) to a so-called carrier network connected to base station 102 typically have certain characteristics in accordance with well-known GSM standards. For example, the signals transmitted between the mobile device and base station represent information using a well-known form of frequency modulation referred to as Gaussian Minimum Shift Keying (GMSK).
For GMSK, output power is normally controlled by a power control, such as an APC, that controls the bias voltage of a power amplifier. This bias voltage is commonly referred to as the VRAMP voltage. Due to the physical nature of the power amplifier the bias voltage is a function of the battery voltage. The battery voltage may be affected by certain conditions that drive the power amplifier into a saturation condition.
There are at least three scenarios in which the power amplifier may be driven toward its saturation condition. A device 104A with a weak battery may limit the power that the device 104A can generate and radiate. Similarly, a device 104B that generates or experiences excessive heat may limit that device's power capabilities. Still further, a device 104C experiencing an instantaneous mismatched antenna load may have an adversely affected battery voltage. Such a mismatched antenna load may occur if the mobile device 104C is placed on a metal plate 122 or table. Under these circumstances, the response of the power amplifier to VRAMP gets smaller. This leads to multiple problems. For example, the APC needs more time to get to the final power level, potentially violating the power-time template GSM requirements, which require the shape of the transmit burst to be within precise parameters. Additionally, most power amplifiers exhibit a strong Amplitude Modulation/Phase Modulation (AM/PM) conversion in the saturating region, which may violate the peak phase error specification. Also, as the power amplifier ramps down VRAMP has first to come out from the saturated region and loses time in ramping from its maximum to zero (or to a very low voltage at which the output power is negligible) in a very short time, which may cause a violation of the switching spectrum requirement. Thus, operating the power amplifier in saturation is not desired.